PROJECT SUMMARY. Human T-cell leukemia virus type 1 (HTLV-1) is a complex retrovirus that is the causative agent of a variety of clinical disorders including adult T-cell leukemia (ATL), an aggressive and often fatal malignancy of mature activated T-cells. ATL is frequently diagnosed after several decades of infection, suggesting that a long period of viral latency contributes to the development of this disease. The HTLV-1 basic leucine zipper factor (HBZ) is believed to play a role in viral latency and T-cell proliferation. This protein is localized in the nucleus and carries a basic leucine zipper (bZIP) domain that promotes protein dimerization through interactions with appropriate leucine zippers in other proteins. Cellular bZIP factors that are bound by HBZ include certain AP-1 transcription factor components (c-Jun, JunB, JunD) and members of the ATF/CREB family (ATF-1, CREB, CREM and CREB-2). Overall, interactions with HBZ are believed to repress transcription by preventing factors from binding the DNA, an effect that is correlated with the atypical basic region in the HBZ bZIP domain. We found that HBZ also binds directly to the cellular coactivators p300 and CBP, which is mediated through regions of the viral protein outside its bZIP domain. In the context of the HTLV-1 promoter, we found that binding of HBZ to both CREB and p300/CBP is essential to achieve full repression of transcription. As the HBZ gene is uniquely encoded on the minus strand at the 3' end of the provirus, its expression is not affected by this and other mechanisms of repression targeting the 5' LTR (the normal viral promoter). It is likely that the binding of HBZ to cellular bZIP factors and p300/CBP underlie its ability to deregulate expression of many cellular genes. To gain insight into the mechanisms by which HBZ affects transcription, we have evaluated its interactions with cellular transcriptional regulators and we have identified genes that it targets. We have obtained evidence that complexes formed between HBZ and ATF/CREB factors are structurally distinct from complexes formed with AP-1 factors. In addition, we found that HBZ directly targets multiple domains of p300/CBP, including the KIX, ZZ and TAZ2 domains. In this proposal we will address the functional consequences of these interactions. In Aim 1, we propose to dissect the interaction between HBZ and p300/CBP, and determine downstream effects of this interaction, including alterations in the abilities of the coactivator to interact with and/or acetylate specific cellular proteins. In Aim 2, we will compare the interaction of HBZ with ATF/CREB and AP-1 proteins. In Aim 3, we will characterize mechanisms of HBZ-mediated deregulation of cellular gene expression. We will test whether abnormal expression of specific genes underlies certain biological aspects of HTLV-1 infection that may relate to development or clinical presentations of ATL.